Thermochromic Perovskite Inks for Reversible Smart Window

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Thermochromic Perovskite Inks for Reversible Smart Window Applications Michele De Bastiani,† Makhsud I. Saidaminov,†,§ Ibrahim Dursun,† Lutfan Sinatra,† Wei Peng,† Ulrich Buttner,‡ Omar F. Mohammed,† and Osman M. Bakr*,† †

KAUST Solar Center (KSC), Division of Physical Science and Engineering (PSE), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia ‡ Nanofabrication Core Lab, King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

S

until an intense orange color was obtained. Subsequently, the solution was cooled while stirring at room temperature to form a dense yellowish mixture. This yellow ink can be heated to a different temperature to observe the chromatic variation (Figure 1a,b). The color of the TC ink changes from yellow

mart windows are technologically advanced windows that can switch the transmittance of incident light between transparent and opaque states.1−3 This emerging technology reduces the energy consumption of buildings and tailors specific camouflage coatings. The working mechanism of smart windows is related to the materials employed, i.e., electrochromism, thermochromism, photochromism, or gasochromism.4−7 Among these, thermochromics (TCs) attracted broad interest due to their simple reversible mechanism of using thermal energy for switching transmittance. Typically, commercially available TCs are liquid crystals8 and leuco dyes,9 and recently vanadium dioxide.10 However, despite the growing interest and potential advantages, state-of-the-art TCs suffer from factors limiting their wide utilization, such as narrow chromatic variation, high operating temperature, and UV-light instability. Hybrid halide perovskites recently received significant attention in photovoltaics,11−17 due to their unique combination of optoelectronic properties including high carrier mobility, long diffusion length, low trap densities,18−23 and power conversion efficiency which is approaching that of crystalline silicon solar cells.24 Besides the great interest in photovoltaics, a growing curiosity has developed in exploiting other unique properties.25−30 Recently, several studies on single crystals have shown that halide perovskites have unusual crystallization behavior; in particular, the solubility of perovskites decreases with the increment of the temperature due to an inverse temperature crystallization (ITC) process that enables the simple growth of perovskite single crystals.31 Interestingly, the crystallization temperature in ITC depends on the halogen constituent of the perovskite. This observation inspired us to combine the tunable optical properties and unusual crystallization processes of perovskites (see below) to design a facile synthesis of TC inks that realize smart windows with wider chromatic variation than thought possible in hybrid perovskites.32 To prepare 1 mL of TC ink, 461 mg of lead iodide (PbI2 from Sigma-Aldrich, purity 96%), 367 mg of lead bromide (PbBr 2 from Sigma-Aldrich, purity 98%), 159 mg of methylammonium iodide (MAI, from Dyesol), and 112 mg of methylammonium bromide (MABr, from Dyesol) were dispersed in a 1 mL mixture of γ-butyrolactone (GBL, from Sigma-Aldrich) and dimethylformamide (DMF, from SigmaAldrich) (1:1 by volume) at 70 °C. The mixture was stirred © 2017 American Chemical Society

Figure 1. (a) Photograph and (b) schematic representation of the perovskite TCs at 25, 60, 90, 120 °C. (c) Absorption spectra of the precipitated solids collected from the respective inks (Panel a is reused with permission from KAUST).

to orange when the temperature reaches 60 °C. In these conditions, the solubility changes together with the initial formation of a fine precipitate. The second chromatic variation from orange to bright red occurs when the temperature is increased further to 90 °C (see Movie S1). Finally, when the temperature reaches 120 °C, the color of the mixture turns gradually from dark red to black. To investigate the optical properties of the perovskite inks, three mixtures were heated to different temperatures (60, 90, and 120 °C) and the colored powders were filtered and collected. Figure 1c shows the absorption spectra of the powders. The extrapolated edges of absorption spectra are 597 nm for the sample heated at 60 °C, 615 nm at 90 °C, and 651 nm at 120 °C. The absorption red shift induced by the increment of the temperature confirms the chromatic variation of the samples. To investigate further the mechanism that drives color change of the TC ink, we performed X-ray diffraction (XRD) of Received: December 1, 2016 Revised: April 4, 2017 Published: April 4, 2017 3367

DOI: 10.1021/acs.chemmater.6b05112 Chem. Mater. 2017, 29, 3367−3370

Communication

Chemistry of Materials

shift of XRD peaks is also consistent with the bandgap change (Figure S2). The selective formation of a single type of perovskite is not only induced by the temperature but also supported by appropriately selected combination of solvents. Indeed, DMF behaves as a good solvent for the inverse crystallization of MAPbBr3, whereas it behaves like a bad solvent for the crystallization of MAPbI3. On the other hand, GBL behaves as a bad solvent for MAPbBr3, whereas it is a good solvent for MAPbI3.35 The overall process is summarized in Scheme 1.

the collected powders. The XRD patterns are depicted in Figure 2a.

Scheme 1. Composition of Inks That Drives the Color

Figure 2. (a) XRD pattern of the solids precipitated at different temperatures and (b) magnification in the low angle region. (c) SEM images of the solids precipitated at different temperatures. The scale bar identifies a length of 10 μm.

This chromatic variation induced by temperature is totally reversible in the presence of the solvents (see Movie S2). During cooling from 120 °C to room temperature, the chemical reactions step-back and the color of the mixture changes from black to red, orange, and finally yellow. This process also restores the original dense opaque precipitation. The kinetics of the heating and cooling processes are reported in Figure 3. The

From the XRD analysis, we observed a gradual shift of the major diffraction peaks to lower angles when the temperature varies from 60 to 120 °C. This indicates a gradual dilation of the MAPbBr3 crystal lattice, due to an incremental increase of the proportion of iodine in the perovskite products.33 Furthermore, we observed that impure satellite diffraction peaks in the low angle range (below 10°) emerged in the lowtemperature products and then tended to disappear in the products as the temperature increased to 120 °C (Figure 2b). Simple precursors, such as methylammonium halide and lead halide, were ruled out as contributors to these satellite peaks (see Figure S1), whereas the small angles of these impure peaks are indicative of the formation of crystalline solids with large unit cells. Therefore, we propose that these peaks originate from perovskite/solvent complexes, as observed previously in MAPbI 3 /DMSO complexes.34 This assumption is also supported by the observation that the impure phases disappeared in the products when the temperature was raised to 120 °C because perovskite/solvent complexes usually produce pure perovskite phase under high temperature annealing with the release of solvent molecules.35,36 The SEM images of the collected powders (Figure 2c) reveal that the size of the crystallites is relatively uniform, with dimensions between 5 and 20 μm, and a cuboidal shape. Initially at room temperature, the precursors are partially dissolved or complexed by the solvents, which makes the mixture dense and cloudy, with a pale yellow color. When the temperature is increased to 60 °C, PbI2 is completely dissolved due to the maximum solubility of MAPbI3.36 The consequence is the liquefaction of the dense mixture to a limpid solution. Meanwhile, MAPbBr2.7I0.3 is formed due to the decrease of solubility of bromide-based perovskite, which shows orange color. By further increase of temperature, the rapid drop of MAPbI3 solubility results in more incorporation of iodine forming red MAPbBr2.4I0.6 at 90 °C and black MAPb-Br1.8I1.2 at 120 °C. The compositions of the perovskites are estimated from the XRD patterns in accordance with Vegard’s law.37 The

Figure 3. Chromatic variation as a function of time and temperature. Hollow dots: heating from 25 °C to the desired temperature. Full dots: cooling from 120 to 25 °C. The dotted fits are a guide for the eye.

time necessary to convert the yellow ink to other specific colors is inversely proportional to the temperature: less than 30 s at 120 °C for the black, 1 min at 90 °C for the red, and no more than 2 min at 60 °C for the orange inks. Under stirring conditions, the cooling dynamics of the mixture takes up to 12 min to restore from black to yellow, with intermediate states of red and orange. Without a stirrer, the time necessary to restore the original colors is extended to several hours due to the limited mass transport (see Figure S3). Inspired by the stable reversibility of the mixture, we realized a TC window prototype. The requirement for the reversible behavior, as mentioned earlier, is the preservation of the solvent mixture ratio of DMF to GBL. To fulfill this requirement, we 3368

DOI: 10.1021/acs.chemmater.6b05112 Chem. Mater. 2017, 29, 3367−3370

Communication

Chemistry of Materials

Figure 4. (a) Picture of the TC prototype at 25 °C (top) and 60 °C (bottom). (b) Absorption spectra of TC prototype after different annealing temperatures: 25 °C yellow, 60 °C orange, and 90 °C red. (c) Band edge variation of the TC prototype. Open circles represent the extrapolated edge at 25 °C; red dots represent the extrapolated edge at 60 °C.

sealed the ink inside the glasses of a customized cuvette. The cuvette is sealed to avoid the evaporation of the solvents, preserving the qualities of the prototype. A picture of the TC prototype at 25 °C and after annealing at 60 °C is reported in Figure 4a. The prototype exhibits a conversion time of just a few seconds during the heating process, independently from the temperature. In contrast, the conversion time during the cooling process requires several minutes and depends on the temperature from which the mixture was cooled from. Interestingly, we noted that the conversion time can be reduced by decreasing the thickness of the prototype (optimal thickness = 1 mm, see Figure S4). We postulate that the favorable heat exchange facilitates the global conversion process. A summary of the conversion times is reported in the Supporting Information (SI). Subsequently, we tested the optical properties of the TC prototype at different annealing temperatures. The absorption spectra are reported in Figure 4b. At room temperature, the prototype exhibits an absorption edge that matches the absorption of solvents and precursor complexes,34 which is yellow colored. Heating at 60 °C changes the color of the perovskite mixture and red-shifts the absorption edge; the color of the prototype changes to a semitransparent bright orange. At 90 °C, the absorption edge extends toward the end of the visible spectrum; the color of the prototype tends to a dark red with some variations between orange and black, a sign that the conversion process is not completely homogeneous. Finally, we tested the stability of the TC prototype, by monitoring the thermochromic behavior over several heating/cooling cycles. In Figure 4c, we reported the extrapolated band edge from the absorption spectra of consecutive cycling between room temperature and 60 °C. Promisingly for applications, the prototype reproduces with high fidelity the thermochromic behavior of the inks. In conclusion, we utilized the unusual crystallization process of perovskites along with their tunable optical properties to design a simple synthesis of inks for TC applications. These inks exhibit a completely reversible chromatic variation ranging from yellow to black as the temperature increases from 25 to 120 °C. These properties pave the way for a new class of smart windows and camouflage coatings with an unprecedented range of color based on hybrid metal halide perovskites.

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Movie S1: Heating process (AVI) Movie S2: Cooling process (AVI) Experimental methods; XRD patterns of MABr and MAI; bandgap calculation; conversion dynamics (PDF)

AUTHOR INFORMATION

Corresponding Author

*Osman M. Bakr. Email: [email protected]. ORCID

Omar F. Mohammed: 0000-0001-8500-1130 Osman M. Bakr: 0000-0002-3428-1002 Present Address §

(M.I.S.) Department of Electrical and Computer Engineering, University of Toronto, Toronto, Ontario M5S 3G4, Canada. Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of King Abdullah University of Science and Technology (KAUST). REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05112. 3369

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DOI: 10.1021/acs.chemmater.6b05112 Chem. Mater. 2017, 29, 3367−3370